The present invention relates generally to electronic ballasts, and more particularly, this invention pertains to an electronic ballast that can be wired to operate multiple lamps.
Typical electronic ballasts have a source of DC power, an inverter, and a resonant circuit. A series resonant tank circuit is commonly used to operate two or more fluorescent lamps connected in series. Further prior art is found in U.S. Pat. No. 5,635,799 issued to Hesterman on Jun. 3, 1997 which discloses a "lamp Protection Circuit for Electronic Ballasts." This patent is hereby incorporated by reference. The disclosure of the patent shows several series-connected two-lamp ballast circuits having a series resonant tank circuit and end-of-lamp-life sensor. The disclosure of this patent has a limitation because if one lamp were to be operated instead of two lamps, the lamp current would go up, and the end-of-lamp-life sensor would not operate properly.
What is needed is a ballast that can sense how many lamps are present, and adjust an inverter control circuit to maintain essentially equivalent operation regardless of how many lamps are connected.